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It has been 4 days since you first posted this question. Although you've got no answer, 4 days must have been more than enough for you to search for oracle SQL Reference manual (either in printed form, on CD or on internet) and find an answer yourself! I would say that those questions you've posted are not realy that important for you as you obviously have not done anything to find an answer (appart for posting it 4 times in this forum)!

And why you didn't get any answers? I'd say because you are asking such a broad questions that either:
- answers are trivially easy to find in basic documentation, so nobody is willing to do this trivial search for you
or
- to answer such a question would demand to copy-paste an entire section or chapter from teh documentation, and again nobody find is very useful to do that here for you

OK, I'll try to answer your questions...
Q1) when 'ON DEMAND' refresh mode is used in materialized view?
A1) Obviously when you don't want the MV to be refrereshed automaticaly, when you want to refresh them when *you* think is the right moment to.

Q2) What is category in 'create OUTLINE...' syntax and why it is used?
A2) I've never used outlines yet so I have no idea. But when I will need them I will surelly look at manuals first and find 99.99% answers to my questions there.

No offence,

Jurij ModicASCII a stupid question, get a stupid ANSI
24 hours in a day .... 24 beer in a case .... coincidence?